Add a Christmas theme

Christmas red window and panels with snow white buttons and red text, a thick
white "fur" trim on popups, cards, the lists and the player bar, the tree's
green for the chosen tab and the play button, and gold for the playing song and
highlights. Icons on buttons take the text's colour, and drop-down lists stay
dark red so their arrow shows. It uses Nord's round shapes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/integration/test_theme_picker.gd

@@ -27,7 +27,7 @@ func test_picker_lists_every_theme_and_shows_the_one_in_use() -> void:
 	for index: int in _settings.theme_button.item_count:
 		names.append(_settings.theme_button.get_item_text(index))
 
-	assert_array(names).contains_exactly(["Classic", "Original web", "Nord"])
+	assert_array(names).contains_exactly(["Classic", "Original web", "Nord", "Christmas"])
 	assert_str(_settings.theme_button.get_item_text(_settings.theme_button.selected)) \
 			.is_equal("Classic")